1. Field of the Invention
The invention relates, in general, to a semiconductor sensor having a base element and at least one deformation element composed of a semiconductor substrate that is doped with a dopant of a first conductivity type. In particular, at least one piezoresistor, doped with a dopant of an opposite conductivity type from the first conductivity type, is located in the deformation element, and the deformation element is in contact with a medium in at least one part of its surface.
A semiconductor sensor is a sensor that can be fabricated using process steps that are known from microelectronics. As a result of the fabrication with process steps that are known from microelectronics it is possible to connect a semiconductor sensor of the generic type to an electronic circuit, or to integrate it into such a circuit. The known semiconductor sensors of the generic type are designed as piezoresistive pressure sensors for absolute, relative, and differential pressure measurements. In order to obtain a high degree of sensitivity, the deformation element, for example a silicon diaphragm, must have the greatest possible extent (edge length or radius) and the greatest possible thickness.
Because of the large superficial extent of the deformation element, the consumption of material and the effort to manufacture it are large. This applies in particular in the case of semiconductor sensors for measuring small pressures or pressure differentials or small changes in acceleration.
2. Summary of the Invention
It is accordingly an object of the invention to provide a generic semiconductor sensor with a deformation element which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type in such a way that the deformation element has the smallest possible superficial dimensions and has the highest possible degree of sensitivity.
This object is achieved in that a semiconductor sensor of the generic type is designed in such a way that a part of its surface has a lower concentration of a dopant than a further region that is located between it and the piezoresistor.
The invention therefore provides for a semiconductor sensor to be produced in which the concentration of the dopant varies between the piezoresistive resistor track and a free surface of the deformation element.
With the foregoing and other objects in view there is provided, in accordance with the invention, a semiconductor sensor that includes a base element and a deformation element made from a semiconductor substrate that is doped with a dopant of a first conductivity type. The deformation element includes at least one part that is in contact with a medium and that has a concentration of the dopant of the first conductivity type. The deformation element is disposed on the base element. At least one piezoresistor is located in the deformation element. The piezoresistor is doped with a dopant of a second conductivity type that is opposite to that of the first conductivity type. The deformation element includes a region having a concentration of a dopant and which is located between the part and the piezoresistor. The part has a concentration of a dopant that is lower than the concentration of the dopant of the region.
The free surface of the deformation element can be in contact with any desired medium. The medium may be either gaseous, liquid, or solid. It makes it possible to deform the deformation element or it acts on the deformation element in such a way that it deforms. This medium may be, for example, one whose properties are to be detected, such as is the case with a pressure sensor. It is equally possible for the medium merely to have the function of permitting the deformation element to be deflected, such as in the case of an acceleration sensor. In both cases, the free surface of the deformation element has a lower concentration of the dopant than at least one region lying between it and the piezoresistor, which is, for example, a piezoresistive resistor track.
The semiconductor sensor according to the invention has a high dielectric strength, which means that the electrical breakdown voltage is increased. Because of the relatively high dielectric strength, a relatively high current can flow through the piezoresistor. The relatively high off-state voltage which is formed in such a case does not lead to an electrical breakdown because of the relatively high dielectric strength. At the same time, the extent of the deformation element can be reduced during its fabrication using an etching method, because an effective etch stop is provided.
It is particularly expedient to design the semiconductor sensor in such a way that the deformation element is of planar design. It is also advantageous if the part of the surface and the further region are formed by layers which extend parallel to the main plane of the deformation element.
Such a semiconductor sensor is characterized by its ease of fabrication. Therefore, while the layers are being grown on epitaxially, it is possible to vary the dopant content between the layers lying one on top of the other by varying the dopant content in the supplied gas atmosphere.
If the exposed part of the surface is bounded by a surface region which adjoins a base element, the concentration gradient between the deformation element and the base element adjoining it is particularly high. During the fabrication of unsupported regions of the deformation element by means of an etching process, for example, an electrochemical deep etching process, this concentration gradient gives rise to an optimum etch stop. One or more upper layers are doped more highly in order to delimit the electrical field between them and the piezoresistive track or tracks.
In accordance with an added feature of the invention, there is provided an embodiment of the semiconductor sensor that is particularly easy to fabricate in which the deformation element has two layers. The first layer forms the part of the surface, the second layer forms the further region, and the second layer adjoins the piezoresistive resistor track.
In accordance with an additional feature of the invention, there is provided a semiconductor sensor in which the thickness of the deformation element is reduced further. It is characterized in that the deformation element has three layers, the first layer forming the part of the surface, the second layer adjoining the piezoresistive resistor track, and the further region being formed by a third layer located between the first and the second layer.
In order to provide a deformation element having the smallest possible thickness and with the highest possible dielectric strength, it is advantageous for the concentration of the dopant in the region with the highest concentration to be higher than in the region with the lowest concentration by at least a factor of 10.
In order to obtain a deformation element having the smallest possible thickness, the thickness of the region with the highest concentration is less than 10 .mu.m.
The semiconductor sensor may be designed as a pressure sensor, which is expediently effected by connecting the deformation element in its edge region to the base element.
In this case, the deformation element is preferably designed as a thin diaphragm. The deformation element can be in the shape of a circular plate, a circular ring-shaped plate with a flexurally rigid center, a rectangular plate, or a rectangular plate with a flexurally rigid center. Each of these plate geometries has specific associated mechanical stress characteristics. The piezoresistive resistor tracks are expediently configured in the regions where a particularly high mechanical structure occurs. As a result, even a small deflection of the pressure sensor gives rise to a detectable electrical voltage.
The semiconductor sensors according to the invention can, however, also form acceleration sensors. This is effected particularly expediently by virtue of the fact that certain sections of the deformation element are in contact with a wall of the base element and a seismic mass is configured on the opposite side of the deformation element. The deformation element is thus only connected to the base element by certain sections. On at least one further section, a seismic mass is connected to the deformation element.
The functional principle of the acceleration sensor is based on the fact that a force which is converted into an electrical signal by means of the sensor mechanism is generated by accelerations acting on a sensor mass from the outside. In this piezoresistive acceleration sensor, the mechanical stress is converted into a change in the resistivity because of the piezoresistive effect. In further deformation regions, the change in resistance is proportional to the acceleration. While an enclosed, thin diaphragm is formed by the deep etching process in the case of a pressure sensor, for this micromechanical acceleration sensor the etching process is carried out in such a way that the deformation element is in the shape of a web.
It is particularly expedient if the exposed surfaces of this web have a low dopant concentration. In this case, the piezoresistive resistors are provided with at least one envelope made of a material with a higher dopant concentration, while the outer surfaces of the web have a lower dopant concentration.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a semiconductor sensor with a base element and at least one deformation element, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.